(1) Field of the Invention
The present invention relates to a technology for an image forming apparatus such as a printer and a copier, and in particular, to a technology for controlling a warm-up operation of a fixing device included in the image forming apparatus, particularly upon commencement of the warm-up operation.
(2) Related Art
Image forming apparatuses, such as a printer and a copier, are provided with a fixing device for heat-fixing an unfixed image formed on a recording sheet onto the recording sheet. For such a fixing device to perform heat-fixing, it is required that the fixing device be warmed up so that the temperature of the fixing device reaches a predetermined target temperature (for instance, 180° C.).
In a commonly-used fixing device including a heating rotating body (a heating roller or a heating belt) and a pressurizing rotating body (a pressurizing roller or a pressurizing belt) that are disposed in a pressure contact state where the pressurizing rotating body contacts and applies pressure to the heating rotating body, warm up of the fixing device is performed while causing the rotating bodies to rotate in the above-described pressure contact state. By warming up the fixing device in such a manner, the fluctuation of the surface temperature of the pressurizing rotating body is reduced, and hence, the surface temperature of the pressurizing rotating body is stabilized. Further, this leads to preventing the occurrence of uneven fixation when printing (image forming processing) is commenced following the completion of the warm up of the fixing device.
In so as to quickly stabilize the surface temperature of the pressurizing rotating body during the warm up of the fixing device, it is desirable that both rotating bodies be caused to rotate at a comparatively high speed. However, when increasing the speed at which the rotating bodies are caused to rotate (for example, when warming up the fixing device while causing both rotating bodies to rotate at a speed equivalent to the speed at which the rotating bodies are caused to rotate during heat-fixing in the image forming processing), the following problems arise. That is, the increase in the speed at which the rotating bodies are caused to rotate leads to an increase in the amount of heat transferring from the heating rotating body to the pressurizing rotating body during the warm up of the fixing device. When an increased amount of heat transfers from the heating rotating body to the pressurizing rotating body during the warm up of the fixing device, the temperature of the heating rotating body does not rise as desired. Due to this, a comparatively great amount of time is required for the surface temperature of the heating rotating body to reach the above-described target temperature, and accordingly, the amount of time required to warm up the fixing device increases.
Such an increase in the amount of time required for warming up the fixing device is particularly problematic in image forming apparatuses capable of performing quick print processing, which have been coming into use recently. An image forming apparatus capable of performing quick print processing cuts down the time from the commencement of the warm up of the fixing device to the commencement of the image forming processing by terminating the warm up of the fixing device and commencing the image forming processing when the temperature of the heating rotating body reaches a print-executable temperature. The print-executable temperature is such that, when the temperature of the heating rotating body reaches the print-executable temperature through the warm up of the fixing device, the temperature of the heating rotating body can reach a fixing temperature when the first recording sheet arrives at a fixing nip of the fixing device. In such an image forming apparatus capable of performing quick print processing, it is desirable that the amount of time required to warm up the fixing device be reduced to as short a time as possible.
In order to cut down the amount of time required to warm up the fixing device, technologies are disclosed, for instance, of causing the rotating bodies to rotate at a slower speed during the warm up of the fixing device compared to the normal speed at which the rotating bodies are caused to rotate during heat-fixing (refer to Japanese Patent Application Publication No. 2000-242126 and Japanese Patent Application Publication No. 2009-265154). By causing the rotating bodies to rotate at a comparatively slow speed during the warm up of the fixing device than during heat-fixing, the amount of heat transferring from the heating rotating body to the pressurizing rotating body during the warm up of the fixing device is reduced, and accordingly, the temperature of the heating rotating body rises at an accelerated rate. As a result, the amount of time required for the surface temperature of the heating rotating body to reach the target temperature is reduced, and accordingly, the amount of time required to warm up the fixing device is reduced.
However, according to the technology disclosed in Japanese Patent Application Publication No. 2000-242126, the warm up of the fixing device is performed while causing the rotating bodies to rotate at a slower speed compared to the normal speed at which the rotating bodies are caused to rotate during heat-fixing. Therefore, when the ambient temperature of the fixing device is low upon commencement of the warm up of the fixing device, there is a risk of the temperature of the heating rotating body reaching the target temperature before the transfer of heat from the heating rotating body to the pressurizing rotating body has progressed to a desired extent (i.e., in a state where a considerable difference lies between the temperature of the heating rotating body and the temperature of the pressurizing rotating body).
If the temperature of the heating rotating body reaches the target temperature while the transfer of heat from the heating rotating body to the pressurizing rotating body has not progressed to a desired extent, a comparatively great amount of heat transfers from the heating rotating body to the pressurizing rotating body when heat-fixing is commenced while causing the rotating bodies to rotate at the normal speed following the completion of the warm up of the fixing device. The transfer of a considerable amount of heat from the heating rotating body to the pressurizing rotating body upon commencement of heat-fixing leads to a decrease in the surface temperature of the heating rotating body. This is problematic, since when the quick print processing is performed under such a situation, there is a risk of the temperature of the fixing device not reaching the fixing temperature before the first recording sheet arrives at the fixing nip of the fixing device. As such, the application of the technology disclosed in Japanese Patent Application Publication No. 2000-242126 may lead to the occurrence of fixing failure.
In addition, when the technology disclosed in Japanese Patent Application Publication No. 2009-265154 is applied, the warm up of the fixing device is performed while causing the rotating bodies to rotate at a slower speed compared to the normal speed at which the rotating bodies are caused to rotate during heat-fixing, similar as in the technology disclosed in Japanese Patent Application Publication No. 2000-242126, and further, heat-fixing is commenced before the temperature of the heating rotating body reaches the fixing temperature and while causing the rotating bodies to rotate at the low speed. Although the application of such a technology has an effect of reducing the amount of time required to warm up the fixing device, the application of this technology may lead to the occurrence of fixing failure for the same reasons as explained above in connection with the technology disclosed in Japanese Patent Application Publication No. 2000-242126. That is, according to the technology disclosed in Japanese Patent Application Publication No. 2009-265154, after the temperature of the heating rotating body has reached the fixing temperature, heat-fixing is performed while causing the rotating bodies to rotate at the normal speed. Due to this, a considerable amount of heat transfers from the heating rotating body to the pressurizing rotating body when the switching from the low speed to the normal speed is performed upon the commencement of the heat-fixing. Accordingly, the surface temperature of the heating rotating body decreases, which may lead to the occurrence of fixing failure.